Method of manufacturing porcelain for electric insulators by controlling grain size

ABSTRACT

THE TOUGHNESS OF PORCELAIN MATERIALS, OR THE CRACKPROPAGATION-RESISTANCE THEREOF; CAN BE IMPROVED BY USING QUARTZ OR CALCINED BAUXITE WITH A CONTROLLED GRAIN SIZE DISTRIBUTION. A METHOD FOR PRODUCING PORCELAINS WITH IMPROVED TOUGHNESS, BY FIRING AT A TEMPERATURE OF 1,180* C. TO 1,350* C., A BODY CONTAINING LESS THAN 30.0% BY WEIGHT OF QUARTZ HAVING A GRAIN SIZE DISTRIBUTION CONSISTING OF LESS THAN 2.0% BY WEIGHT OF PARTICLES WITH AN EFFECTIVE DIAMETER SMALLER THAN 10 MICRONS AND MORE THAN 97.0% BY WEIGHT OF PARTICLES WITH AN EFFECTIVE DIAMETER SMALLER THAN 50 MICRONS, OR A BODY CONTAINING 10.0 TO 60.0% BY WEIGHT OF CALCINED BAUXITE GRAINS HAVING A GRAIN SIZE DISTRIBUTION CONSISTING OF LESS THAN ON THE ORDER OF ABOUT 3% BY WEIGHT OF PARTICLES WITH AN EFFECTIVE DIAMETER SMALLER THAN 1 MICRON AND LESS THAN 60.0% BY WEIGHT OF PARTICLES WITH AN EFFECTIVE DIAMETER SMALLER THAN 10 MICRONS AND MORE THAN 97.0% BY WEIGHT OF PARTICLES WITH AN EFFECTIVE DIAMETER SMALLER THAN 60 MICRONS.

y 1972 NOBORU HlGUCHl ETAL 3,674,519

METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BY CONTROLLINGGRAIN SIZE Filed Aug. 29, 1969 6 Sheets-Sheet 1 2 3 5 7 2025046563100200 Effective diameteript of quartz material July 4, 1972 NQBQRU HlGUCH]ETAL 3,674,519

METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BY CONTROLLINGGRAIN SIZE 6 Filed Aug. 29, 1969 Sheets-Sheet 2 a) p I 0' Illllllll'llllL 040.50.? I 2 345 7 I0 20 30405070|00 Effective diameterw) ofcalcined bauxite and alumina material July 4, 1972 NOBORU HIGUCHI ET AL3,674,519

METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BY CONTROLLINGGRAIN SIZE J Filed Aug. 29, 1969 e Sheets-Sheet 5 2 3 45 7 IO 203040-507OIOO 200 Effective diameter 1) of siliceous sand particles y1972 NOBORU HIGUCHI ETAL 3,674,519

METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BY CONTROLLINGGRAIN SIZE Filed Aug. 29, i969 5 51 41 4 Fig.38

Percent by weight 2 3 45 7 IO 2030405070|00 Effective diometerquofcalcined bauxite 4 1972 NOBORU HIGUCHI ETAL 3,674,519

July METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BYCONTROLLING GRAIN SIZE Filed Aug. 29, 1969 6 Sheets-Sheet 5 w f .L I O 2m I I O I .L L i7 0 l DOA @5125 25? 3 2% wwwwwwowww www t l I I I l I lII a 876543%HD9 WHEN M n m iii n r. E r If \I \I m% M mu .m. m m. 2m umi .n U H2 SNWM C .Inmmwm hm n 9 uv. m m v m n" Wm w m we m 6 ne M n w mmm mm v m m m w m mnw mm A SC Q vo Poqceloin mode of siliceous sand withgrain size distribution of the invention(l|g3A Curve 2) ---Porce|ainmode of siliceous sand F /'g 4 with coarse grain size distribution ofthe invention(Fig3A Curve 3) Porcelain mode of siliceous sand with knowngrain size distribution (Fig 3A curve I) y 1972 NOBORU HIGUCHI ETAL3,674,519

METHOD OF MANUFACTURING PORCELAIN FOR ELECTRIC INSULATORS BY CONTROLLINGGRAIN SIZE Filed Aug. 29, 1969 6 Sheets-Sheet 6 Fig. 5-

Alumina or calcined bauxite content Toughness l7 Parameter :2 2 Q E I4:I3 3 lxlokycm Young s modulus: E B 9- (XIO m 8- --0 Fracture surtaceenergy :l'e ,0 .g::: ;::v:, -"A f:;:: 9- 4*: 4:---

I800 Modulus of rupture of 700' Test pieces (Unglazed :300

388' l i200- ilOO- i500- vltrityilzam I400 .mP.ru c l 200 0 o Porcelaincontaining calcined bauxite with a grain elze distribution according tothe invention Known alumina Porcelain For eian a mo in calcined grafn eze sletri uaon than tha t o t e irwel ti n Porcelain containing calcinedbauxite with a coareer grain size distribution than that at theinvention United States Patent US. Cl. 106-46 Claims -ABSTRACT OF THEDISCLOSURE The toughness of porcelain materials, or thecrackpropagation-resistance thereof, can be improved by using quartz orcalcined bauxite with a controlled grain size distribution. A method forproducing porcelains with improved toughness, by firing at a temperatureof 1,180 C. to l,350 C., a body containing less than 30.0% by weight ofquartz having a grain size distribution consisting of less than 2.0% byweight of particles with an effective diameter smaller than 10 micronsand more than 97.0% by weight of particles with an effective diametersmaller than 50 microns, or a body containing 10.0 to 60.0% by weight ofcalcined bauxite grains having a grain size distribution consisting ofless than on the order of about 3% by Weight of particles with aneffective diameter smaller than 1 micron and less than 60.0% by weightof particles with an effective diameter smaller than 10 microns and morethan 97.0% by weight of particles with an efie'ctive diameter smallerthan 60 microns.

This invention relates to a method for manufacturing porcelain forelectric insulators, and more particularly to a method of manufacturingporcelain for electric insulators having an excellent resistance againstcrack propagation.

Feldspathic porcelain and alumina porcelain are known as porcelainmaterial for electric insulators. The feldspathic porcelain belongs toquartz-feldspar clay system porcelains, which essentially consist of 20to 40% by weight of feldspathic material, 15 to 30% by weight of quartzmaterial, and 40 to 60% by weight of clay material. The feldspathicporcelain is featured in its low material cost and its excellentelectric insulation, and it is very widely used for insulators. However,the mechanical strength of the feldspathic porcelain is rather low,e.g., its bending strength is about 800 to 1,100 kg./cm. and hence, ithas not been used for high-voltage insulators requiring high mechanicalstrength. The alumina porcelain belongs to alumina-feldspar-quartz-claysystem porcelains, which essentially consist of 10 to by weight ofalumina, 20 to 40% by weight of feldspathic material, less than 30% byweight of quartz material, and 20 to 60% by weight of clay material. Thealumina porcelain is featured in its high mechanical strength. In fact,a test piece of the alumina porcelain has proved to have a maximumbending strength of about 1,800 kg./cm.

ICC

Accordingly, the application of the alumina porcelain to high-voltageinsulators has been expanding. The alumina porcelain, however, hasdrawbacks in that its material cost is high and it requires a highvitrifying temperature.

It is well known that a suspension insulator has a metallic cap and ametallic pin, each cemented to a porcelain insulating body at its outertop end and at itS inner lower end, respectively. Thus, a plurality ofsuspension insulators can be strung as a string of insulators 'byconnecting the pin of each suspension insulator to the cap of the nextlower insulator in succession. The porcelain body has a shade portionwith a rib, which is formed by corrugating or waving the lower surfaceof the shade portion, so as to provide the surface leakage distancenecessary for each insulator.

It has been frequently reported during the last two decades thatsuspension insulators are broken by the shooting of mischievous hunters,who take the suspension insulators for targets. When the suspensioninsulator is obliquely shot from below by a rifle bullet, the rib is hitand the shade portion is crashed down in pieces, and a crack is formedextending from the crashed surface along the radial direction of thesuspension insulator (to be referred to as a radial crack, hereinafter).If the radial crack propagates toward the head portion of the insulatorin excess of the border between the metallic cap and the shade portion,the insulating strength of the insulator is so lowered that it cannotwithstand the service voltage or an over-voltage applied across theinsulators. Thus, there is caused an internal flashover along the thuspropagated radial crack. The internal flashover produces an extremelyhigh temperature to burn and gasify the material sealed within themetallic cap, such as paint, cork, cement, a metallic pin, etc. As aresult, an explosive pressure is generated within the metallic cap, sothat the porcelain, the cement, and the metallic pin are separatedeasily propagates. In order to minimize the length of the radial crack,various design modifications have been proposed, such as elongation ofthe head portion of the porcelain body and increasing the thickness ofthe shade, such modifications have, however, proved to be ineffective.There has been made some improvement in the mechanical strength andelectrical insulating strength of feldspathic porcelain, e.g., U.S. Pat.No. 3,097,101, in which the porcelain material consists of clay, fluxmaterial, and flint particles whose grain size is l to 45 microns dia.This US. patent is a development from studies of the effect of quartz onthe mechanical strength of feldspathic porcelain.

On the other hand, the nature and behavior of the resistance againstcrack propagation (to be referred to as "crack-propagation-resistance,hereinafter) of porcelain insulators are entirely irrelevant to those ofthe mechanical strength and electric insulating strength thereof. Thus,the aforementioned developments in the mechanical strength and theelectric insulating strength do not provide any substantial contributionto improvement of the crack-propagation-resistance. In fact, there hasnot been made any significant improvement in thecrack-propagation-resistance of the porcelain, i.e., the fracturetoughness (toughness).

Therefore, an object of the present invention is to provide a method formanufacturing porcelain for electric insulators, which has an excellentcrack-propagationresistance.

Another object of the present invention is to provide a method formanufacturing inexpensive porcelain for electric insulators, which hasexcellent electric characteristics and excellent mechanical strength.

.A further object of the present invention is to provide a method formanufacturing porcelain for electric insulators, which can hold powerline conductors, even when an excessively high mechanical shock isapplied thereto, so as to mitigate the risk of the falling of power lineconductors.

A still further object of the present invention is to provide a methodfor manufacturing porcelain for electric insulators, in which thedistribution of the grain size of raw materials in the porcelain body iscontrolled.

A still further object of the present invention is to provide a methodfor manufacturing porcelain for electric insulators, in which thedistribution of the grain size of raw materials in the porcelain body isrestricted to a specific range, which is coarser than that of the priorarts.

A still further object of the present invention is to provide aporcelain for electric insulators having an excellentcrack-propagation-resistance, which porcelain includes polycrystallineaggregates of crystals.

The foregoing objects and other objects as well as the characteristicfeatures of the invention will become more apparent and more readilyunderstandable by the follow ing description and the appended claimswhen read in conjunction with accompanying drawings, in which:

FIG. 1 is a front elevation of a suspension insulator, with a halfthereof shown in section;

FIG. 2A is a graph showing the grain size distribution of quartzmaterial usable in the method according to the present invention, incomparison with the corresponding grain size distribution of quartz,material used in known porcelain;

FIG. 2B is a graph showing the grain size distribution of calcinedbauxite usable in the method according to the present invention, incomparison with the corresponding grain size distribution of aluminamaterial used in known alumina porcelain;

FIG. 3A is a graph showing the grain size distribution of quartzmaterial used in an embodiment of the present invention;

FIG. 3B is a graph showing the grain size distribution of calcinedbauxite used in an embodiment of the present invention;

FIG. 4 is a diagrammatic illustration, showing how various properties ofporcelain containing different amounts of alumina are affected by thecontent and grain size of quartz material used in the porcelain; and

FIG. 5 is a diagrammatic illustration, showing how various properties ofporcelain are affected by the content and grain size of alumina orcalcined bauxite used in the porcelain.

In the following description, a mixture of raw materials for porcelainproduction will be referred to as a body, and the body which is readyfor shaping after being refined, dehydrated and kneaded will be referredto as a plastic body.

In an embodiment of the present invention, there is provided a methodfor manufacturing feldspathic porcelain, wherein a feldspathic body isprepared by mixing 30.0 to 50.0% by weight of finely pulverizedfeldspathic material, which is commercially available feldspathic powderor made by mechanically crushing raw feldspar followed by wet finepulverization with a trommel, or made by further pulverizing thecommercially available feldspathic powder; 10.0 to 30.0% by weight ofquartz material, less than 2.0% by weight of the total quartz materialbeing particles with an eifective diameter smaller than 10 microns,while more than 97.0% by weight of the total quartz material beingparticles with an effective diameter smaller than 50 microns, saidquartz material being made by pulverizing raw quartz material with apulverizer, e.g., a vibrating mill, or by classifying commerciallyavailable quartz particles, e.g., siliceous sand, with a dry classifieror the like; 30.0 to 60.0% by weight of clay material, e.g., Gairomeclay (clay available in Japan, mainly consisting of quartz and kaolin),Kibushi clay (clay available in Japan, mainly consisting of fineparticles of kaolin), China clay, ball clay, kaolin, etc.; and theremainder of water. Special care is taken in the mixing process not tochange the grain size distribution of the quartz material. For this end,a suitable mixer, like a trommel, is used for the mixing. The slurry ofthe body thus prepared is passed through a screen and a ferrofilter, soas to remove the impurities, such as sawdust, wooden chips, iron chips,etc. The slurry is then dehydrated into a cake by removing excessmoisture by a suitable means, such as a filter press, so that the cakecan be kneaded by a kneading machine, e.g., a deairing pugmill, toproduce a plastic body. The plastic body is dried, glazed, and fired ata temperature of l,l C. to 1,350 C. in a furnace, e.g., a tunnel kiln.

As another embodiment of the present invention, there is provided amethod for manufacturing alumina porcelain in a manner similar to thatfor the manufacture of the feldspathic porcelain by using an aluminaporcelain body comprising 20.0 to 40.0% by weight of feldspathicmaterial, 5.0 to 25.0% by weight of quartz material, less than 2.0% byweight of the quartz material being particles with an effective diametersmaller than 10 microns while more than 97.0% by weight thereof beingparticles with an effective diameter smaller than 50 microns; 20.0 to55.0% by weight of clay material; and 10.0 to 45.0% by weight of aluminamaterial in the state as prepared by the Bayers process or in the stateas pulverized.

In another embodiment of the present invention, there is provided amethod for manufacturing bauxite-containing porcelain in a mannersimilar to that for the manufacture of the feldspathic porcelain, byusing a bauxitecontaining body comprising 20.0 to 40.0% by weight offeldspathic material, up to 25.0% by weight of quartz material, 2 0.0 to55.0% by weight of clay material, and 10.0 to 60.0% by weight ofcalcined bauxite, in which less than on the order of about 3% by weightof the calcined bauxite is particles with an effective diameter smallerthan 1 micron and less than 60.0% by weight of the total calcinedbauxite is particles with an effective diameter smaller than 10 micronswhile more than 97.0% by weight of the total calcined bauxite isparticles with an effective diameter smaller than 60 microns. Thecalcined bauxite is prepared by crushing raw bauxite, for instancebauxite of the composition as shown in Table 1, into pieces with adiameter of 30 mm. or less, calcining the bauxite thus crushed at atemperature of l,350 C. to l,600 C. in a furnace, e.g., a rotary kiln,so as to produce the so-called calcined bauxite by removing water ofcrystallization and organic compounds in the raw bauxite, pulverizingthe calcined bauxite by a pulverizer, e.g., a trommel and classifyingthe pulverized bauxite into the aforesaid grain size distribution. Duecare must be taken in the mixing of the ingredients of the aforesaidmixture lest the grain size distribution of the pulverized bauxiteshould be changed.

In the method according to the present invention, it is also possible touse a body comprising 20.0 to 40.0% by weight of feldspathic material;up to 25.0% by weight of quartz material prepared by the aforesaidprocess, less than 2.0% by weight of the total quartz material beingparticles with an effective diameter smaller than 10 microns while morethan 97.0% by weight of the quartz material being particles with adiameter smaller than 50 microns; 20.0 to 55.0% by weight of claymaterial; and 10.0 to 60.0% by weight of calcined bauxite prepared bythe aforesaid process, less than on the order of about 3% by weight ofthe calcined bauxite being particles with an efiective diameter smallerthan 1 micron and less than 60.0% by weight of the total calcinedbauxite being particles with an effective diameter smaller than 10microns while more than 97.0% by weight of theitotal calcined bauxitebeing particles with a diameter smaller than 60 microns. In preparingthe body, due care must be taken lest the grain size distribution of thequartz material and the calcined bauxite should be changed.

The composition of the product porcelain obtained by using calcinedbauxite according to the present invention consists of 26.5 to 65.0% byweight of SiO 24.5 to 68.1% by Weight of A1 0.1 to 1.8% by Weight of FeO' 0.3 to 2.4% by Weight of TiO 0.03 to 0.4% by weight of CaO, 0.01 to0.2% by weight of MgO, 2.0 to 6.5% by weight of K 0 and 0.5 to 4.5% byWeight of Na O.

If the quartz material with the aforesaid grain size distribution isincluded in a feldspathic body or. an alumina body, the quartz particlesare distributed in the vitreous matrix of the porcelain made of suchbody. If the calcined bauxite is included in the body, there areproduced polycrystalline aggregates dispersed in the vitreous matrix ofthe porcelain, which polycrystalline aggregates consist of corundum andmullite (aggregate of A1 0 and 3Al O -2SiO in the state of particles).In the porcelain according to the present invention, with such quartzparticles or polycrystalline aggregates dispersed in the vitreous matrixthereof, it a crack is generated, for instance by a rifle bullet, thecrack can propagate with'the microscopic linearity through the vitreousmatrix, until encountering the quartz particles or the polycrystallineaggregates. When encountering the quartz particles or thepolycrystalline aggregates, the cracks cease to propagate in the stateas branched in a number of directions, while scattering or losing thecracking energy, because thecrystalline structure of the quartzparticles or the polycrystalline aggregates of corundum and mullitecannot be cracked by hitting with rifle bullets or the like.

Upon encountering with the quartz particles or the polycrystallineaggregates of corundum and mullite, if the cracks continue to propagatepreferentially in such a manner that it avoids the quartz particles orthe polycrystalline aggregates of corundum and mullite, without beingbranched, the path of the cracks is elongated and thecrack-propagation-resistance increases to reducethe cracking energy.

In short, the quartz particles or the polycrystalline aggregates ofcorundum and mullite act as barriers to the propagation of crack, sothat the cracks in the vitreous matrix of theporcelain become hardtopropagate, and propagation of the cracks ceases at the quartz particlesor the polycrystalline aggregates of corundum and mullite.

It is found that the quartz particles, or the mullite contained in thepolycrystalline aggregates of corundum and mullite, can dissolve in thevitreous matrix of the porcelain surrounding the particles or theaggregates, and such dissolution of the quartz particles or the mullitein the polycrystalline aggregates of corundum and mullite'tends tochange the composition of the vitreous matrix so as to allow easypropagation of the cracks therethrough. According to the presentinvention, such dissolution of the quartz particles or the mullite inthe polycrystalline aggregates of corundum and mullite is minimized byusing quartz material and calcined bauxite material with a grain sizelarger than the corresponding grain size of known porcelaincompositions, whereby a large amount of the quartz the vitrification ofporcelain at a low temperature. As a result, the porcelain can be firedat a comparatively low temperature, even when a considerably largeamount of substance tending to deteriorate the sinterability iscontained in the starting material. In other Words, due to themineralizing action of the titanium dioxide, the aforesaidpolycrystalline aggregates can be contained in the fired porcelain atthe desired high rate, despite the large grain size of the calcinedbauxite and the comparatively low firing temperature at a temperature ofl,l C. to l,350 C., as defined in the foregoing.

Any commercially available materials can be used in the method accordingto the present invention, such as the feldspathic material, the quartzmaterial, the clay material, the alumina material, raw bauxite, and soon. Table 1 shows some examples of the starting material usable in themethod according to the present invention.

TAB LE 1 [In percent by weight] Starting material Feld- Raw Componentsspathic Quartz Clay Alumina bauxite Ignition 1oss.- 1. 5 1. 3 10. 0-16.0 1. 0 11. 0-33. 0 S102 65 0-78. 0 97. 0 42. 0-59. 0 1. 0 0. 5-8. 0

In the method according to the present invention, the firing temperatureis selected to be a temperature of 1,180 C. to 1,350 C., because thefiring at a temperature lower than 1,180 C. results in an incompletesintering, while firing at a temperature higher than 1,350 C. causesexcessive dissolution of the quartz particles or the mullite in thepolycrystalline aggregates of corundum and mullite.

In the method according to the present invention, the grain sizedistribution of the quartz material is selected to consist of less than2.0% by weight of particles with an elfective diameter smaller than 10microns and more than 97.0% by weight of particles with an effectivediameter smaller than 50 microns, because of the use of small particlesin excess of the aforesaid grain size distribution can cause dissolutionof the quartz particles, while the use of large particles in excess ofthe aforesaid grain size distribution improves the toughness ofporcelain as compared with that of known porcelain but deteriorates themechanical strength of the porcelain as compared with that of knownporcelain.

The content of quartz material in the method according to the presentinvention is selected to be 10.0 to 30.0% by Weight in feldspathic body,5.0 to 25.0% by weight in alumina body, and up to 25.0% by weight withthe aforesaid specific grain size distribution in bauxite-containingbody, because if the content of quartz material of the aforesaid grainsize is less than 10.0% by weight in feldspathic body or less than 5.0%by weight in the alumina body, the crack propagation cannot beprevented, while if the content of quartz material of the aforesaidgrain size exceeds 30.0% by weight in the feldspathic body or 25 .0% byweight in the alumina body or bauxitecontaining body, the sintering ofthe body becomes difficult.

In the bauxite-containing body according to the present invention, ifthe grain size distribution of the quartz material is not specified, thecontent of quartz material is selected to be up to 25.0% by weight,because the quartz material in excess of 25 .0% by weight makes thesintering of the body difficult.

In the bauxite-containing body according to the present invention,calcined bauxite is used, because the ignition loss of raw bauxiteamounts to 10.0 to 30.0% by weight. Unless such ignition loss isprecluded, there will be an excessively large firing shrinkage in theproduct, and the accuracy of the product will be deteriorated.

The calcined bauxite usable in the method according to the presentinvention is specified to consist of less than on the order of about 3%by weight of particles with an effective diameter smaller than 1 micronand less than 60.0% by weight of particles with an eifective diametersmaller than microns and more than 97.0% by weight of particles with aneffective diameter smaller than 60 microns, and the content of thecalcined bauxite in the bauxite-containing body is selected to be 10.0to 60.0% by weight, because if more than on the order of about 3% byweight of the calcined bauxite has an effective diameter smaller than 1micron and more than 60.0% by weight of the calcined bauxite has aneffective diameter smaller than 10 microns and if the total content ofthe calcined bauxite in the bauxite-containing body is less than 10.0%by weight, the amount of the polycrystalline aggregates available fordeterring the crack propagation becomes too small and the size ofindividual polycrystalline aggregates becomes too small to achievesatisfactory effects of preventing crack propagation, while the use ofexcessively coarse calcined bauxite containing more than 97.0% by weightof particles with an effective diameter smaller than 60 micronsdeteriorates the mechanical strength and the addition of a too largeamount of calcined bauxite in excess of 60.0% by weight makes thesintering difficult.

In the method according to the present invention, the content offeldspathic material is selected to be 30.0 to 50.0% by Weight in thefeldspathic body or 20.0 to 40.0% by weight in the alumina body orbauxite-containing body, because less than 30.0% by weight offeldspathic material in the feldspathic body or less than 20.0% byweight of feldspathic material in the alumina body or bauxite-containingbody makes the sintering diflicult, while more than 50.0% by weight offeldspathic material in the feldspathic body or more than 40.0% byweight of feldspathic material in the alumina body or bauxitecontainingbody causes excessive melting of quartz particles or the mullite in thepolycrystalline aggregates of corundum and mullite.

The content of clay material usable in the method according to thepresent invention is selected to be 30.0 to 60.0% by weight in thefeldspathic body or 20.0 to 55.0% by weight in the alumina body orbauxite-containing body, because if the clay material in the body isless than or more than the aforesaid range, the body becomes hard toshape.

In the method according to the present invention, the content of aluminais selected to be 10.0 to 45.0% by Weight in the alumina body, becausethe alumina content less than 10.0% by Weight deteriorates themechanical strength of the product, while the alumina content in excessof 45.0% by weight makes the sintering diflicult.

In general, it has been a practice in ceramic industries to express thegrain size distribution of raw materials in percentage by weight ofparticles with an effective diameter smaller than a certain value. Theunit commonly used for indicating the magnitude of the particle diameteris micron FIG. 2A is a graph illustrating the grain size distribution ofquartz material according to the present invention, in comparison withthe corresponding grain size distribution of quartz material in knownfeldspathic body or known alumina body. The abscissa of FIG. 2Arepresents the effective diameter of quartz particle, While the ordinaterepresents the percentage by weight of those particles whose effectivediameter is smaller than a certain value. In FIG. 2A, the rangesurrounded by curves S and S shows the grain size distribution of aknown quartz material, which has been commonly used in the porcelain forelectric insulators, and the range surrounded by curves a and a showsthe grain size distribution of another quartz material, which wasdisclosed by U.S. Pat. No. 3,097,101. On the other hand, the grain sizedistribution of the quartz material according to the present inventionlies in the range surrounded by curves I and I As can be seen from FIG.2A, the grain size distribution of the quartz material, according to thepresent invention, is in a considerably coarser range of effectivediameter than that of commonly used known quartz material.

The grain size distribution of the quartz material in the rangesurrounded by the curves I and I is tabulated in Table 2.

In measuring the grain size distribution, the sedimentation process isused.

TABLE 2 I II III Percentage by weight of particles with an effectivediameter smaller than the value in column I FIG. 4 shows properties ofporcelain prepared by using bodies containing alumina at a rate of 0,10.0, 20.0, or 30.0% by weight; and quartz materials (siliceous sand)with the grain size distributions, as shown in FIG. 3A,

TABLE 3 I II III IV Percentage by weight of particles with an effectivediameter smaller than the value in column I Grain size Grain size Grainsize distribution distribution distrlbution curve 1 curve 2 curve 3Efiective diameter (Fig. 3A) (Fig. 3A) (Fig. 3A)

The grain size distribution of quartz material of the column II of Table3 (corresponding to the curve 1 of FIG. 3A) is in the known grain sizedistribution range of quartz materials, which have been commonly used inporcelain for electric insulators, and the grain size distribution ofthe column II lies within the range surrounded by the curves S and S inFIG. 2A. The grain size dis tribution of quartz material of the columnIII of Table 3 (corresponding to the curve 2 of FIG. 3A) is in the rangeaccording to the present invention, which is shown in FIG. 2A assurrounded by the curves I and 1 The grain size distribution of quartzmaterial of the column IV of Table 3 (corresponding to the curve 3 ofFIG. 3A) lies in a range coarser than that according to the presentinvention, which is shown in FIG. 2A as surrounded by the curves I and IIn FIG. 4, the solid line curves, the dash line curves, and the dash-dotline curves represent properties of porcelains made by using siliceoussands with grain size distribution of columns II, III, and IV of Table3.

The inventors made a series of studies on the scale representing thecrack-propagation-resistance to suppress the propagation of cracksleading to the mechanical breakdown of the porcelain, and as a result, atoughness parameter was selected based on the fundamental theory ofcrack propagation, which is equivalent to the square root of two timesthe product of the fracture surface energy w and the Young's modulus Eof the porcelain, i.e., flx -E. In other words, in FIG. 4, the largerthe toughness parameter /2)\ -E is, the larger thecrack-propagation-resistance of the porcelain is, i.e., the toughter theporcelain is.

The measurement of various properties of sample porcelains of FIG. 4 wascarried out as follows. The Youngs modulus was measured by the Ewingsmethod with 8 mm. wide, 1.5 mm. thick, and mm. long samples. Thetoughness parameter /2'y 'E was measured by the Gilmans doublecantilever beam method, which was described by J. I. Gilman for themeasurement of single crystal, as described in Journal of AppliedPhysics, vol. 31, No. 12, p. 2208. Therefore, w could be calculated fromthe value of E and /2'y -E. According to the Nakayamas Work of fracturemethod, as described in Journal of American Ceramic Society, vol. 48,No. 11, p. 583, the fracture surface energy 'y can be measured byitself. The modulus of rupture was measured by the center point loadingmethod with 300 mm. span cylindrical test pieces of 30 mm. dia. Thevitrifying temperature was determined by firing the shaped bodies ofmaterials with a 30 mm. dia. in a furnace while increasing the firingtemperature at a rate of C. per hour, and the fired bodies werewithdrawn from the firing furnace at different temperatures, and thewater absorption of each sample thus fired was measured, and thetemperature corresponding to the sample with a water absorption of 0%was taken as the vitrifying temperature.

It is apparent from FIG. 4 that the toughness of porcelain made by usingquartz material having a finer grain size distribution than that of themethod according to the present invention is low. On the other hand,when quartz material having a coarser grain size distribution than thatof the present invention, the toughness can be retained at a similarlevel to that of the present invention, while the modulus of rupture isdrastically reduced. It can be also seen from FIG. 4 that the vitrifyingtemperature becomes higher as the grain size distribution of quartzmaterial becomes coarser, as the content of the quartz materialincreases, and as the content of alumina increases, respectively.

FIG. 2B illustrates the grain size distribution (as surrounded by curvesS and S of alumina used in a known alumina body, in comparison with thegrain size distribution (as surrounded by curves I and I of calcinedbauxite used in a bauxite-containing body usable in the method accordingto the present invention.

FIG. 5 shows in dash lines, properties of the porcelain containing thecalcined bauxite with the aforesaid grain size distribution, accordingto the present invention. In the figure, dash-dot lines represent theproperties of porcelain containing calcined bauxite with a finer grainsize distribution than that of the present invention, while dashtWo-dotslines represent the properties of porcelain containing calcined bauxitewith a coarser grain size distribution than that of the invention. Theproperties of porcelain made of a body containing alumina with a grainsize distribution corresponding to that of alumina in known porcelainbody for electric insulators, are shown by solid lines.

The grain size distribution of calcined bauxite, as shown in FIG. 2Bbetween curves I and I is tabulated in Table 4.

The properties, as shown in FIG. 5, were measured in the same manner asthose of FIG. 4.

Referring to FIG. 5, properties of diiferent porcelain containingcalcined bauxite of dilferent grain size distributions at the same ratewill now be compared. The porcelain containing the calcined bauxite withthe grain size distribution according to the present invention has anexcellent toughness as well as outstanding modulus of rupture. Theporcelain containing alumina with a known TABLE 4 I II III Percentage byweight of particles with an effective diameter smaller 12 aforesaidcoarser calcined bauxite is much smaller than that of known aluminaporcelain.

The vitrifying temperature of the porcelain containing the calcinedbauxite increases as the grain size distribution of the calcined bauxitebecomes coarser, and as the than the value in column I 5 Efiective Limitfar coarse Limit for fine content of the calcined bauxite or aluminaincreases. diameter (:1) distribution dist i ti When the grain sizedistribution of the calcined bauxite 0 becomes coarser than that of thepresent invention, the 0 3 vitrifying temperature of the porcelain madeby using ca g 10 the bauxite becomes higher than that of known alumina 5porcelain. In fact, the vitrifying temperature of the porcelaincontaining 40.0% by weight of calcined bauxite with 42.0 79.0 o gg thecoarser grain size distribution is about 1,380 C. as shown in FIG. 5.Since the vitrifying temperature of the 75.0 100.0 o 8M 10M porcelainexceeds 1,350 C., the addition of such coarse 83 6 calcined bauxite doesnot contribute to the improvement 10010 0 of the toughness.

The method of the present invention will now be described in furtherdetail referring to examples. Table 5 grain size distribution has amodulus of rupture subtabulates the chemical composition of startingmaterials stantially comparable with that of the porcelain acused in thefollowing examples.

TABLE 5 I II III IV V VI Item Feldspar Siliceous Gairome clay AluminaCalcined (feldspathic sand (clay (alumina bauxite. material). (quartzmaterial). material).

material). Place of production Kanamaru, U.S.A Giiu-ken, Demerara,

Niigata- Japan. British ken, Japan. Guiana,

South America. Trademark JASPER Raw material Commercially availablecalcined alumina.

Chemical composition (percent by weight):

Ignition loss 0. 5s 0. 10 13. 0. l8 0. 30 S102 07. 95 e9. 82 4s. 89Trace 4. 20 17. 22 0. 10 as. 73 99. 47 91. 23 0. 14. 0. 02 1. 54 0.02 1. 07 0. 01 0. 98 Trace 2. 90 0. 15 Trace 0. 33 0. 05 0. 14 0. 02Trace 0. 21 Trace O. 05 10. 59 0. 01 0. 75 o. 02 0. 04 3.35 0.10 0.120.24 0. 04

cording to the present invention, while its toughness is much smallerthan that of the invention. The porcelain containing calcined bauxitewith a finer grain size distribution than that of the invention has amodulus of rupture of the magnitude comparable with that of theinvention, but its toughness is considerably smaller than that of theinvention. The porcelain containing calcined bauxite with a coarsergrain size distribution than that of the invention has a toughness,which is slightly inferior to that of the invention, as long as thecontent of the calcined bauxite is less than 30.0% by Weight, while asthe content of the calcined bauxite increases in excess of 40.0% byweight, the toughness of the porcelain is greatly reduced, as comparedwith that of the present invention. The modulus of rupture of theporcelain containing the EXAMPLE 1 Feldspathic porcelains were prepared,with the composition according to the present invention and with a knowncomposition, as tabulated in Table 6.

Bodies consisting of the ingredients of Table 6 were shaped and fired at1,250 C. to produce porcelain test pieces. The same bodies were alsoused in producing suspension insulators by firing at 1,250 C. Variousproperties of the porcelain test pieces and the suspension insulatorswere measured, and the results are shown in Table 6. The values of theamount of crystals in Table 6 were obtained by a quantitative analysisby X-ray diffraction. The depth of radial cracks caused by shooting wasmeasured by repeatedly shooting ribs 60 to 6d of each loaded suspensioninsulator, as shown in FIG. 1, under the following conditions until theshade 5 partially breaks down.

TABLE 6 Porcelain Known oi the Items porcelain invention Raw materialcomposition (percent by weight):

Feldspar 40. 40. 0 Siliceous sand 1 20. 0 0. 0 Siliceous sand with acontrolled grain size distribution 2 0. 0 20.0 Gairome clay 40.0 40. 0

Total 100. 0 100. 0

Properties of porcelain:

Porcelain test pieces:

Thougness parameter: IZ' -E (X10 -.'t g.-cm. 10.6 12. 4 Young's modulus:E (X10 g.-cm.- 6. 6 6. 7 Fracture surface energy: Ye (g.-cm.-

cm.- 8. 6 11. 4 Modulus of rupture (unglazed) (kg./

cmfl) 940 970 Amount of crystals (percent by weight) Mullite 21 19 8 16Suspension insulators:

Tensile strength of sound insulator :11 g. 17, 300 17, 400 Depth ofradial crack caused by shooting (mm. 26 8 Tensile strength of insulatorsafter shooting test n (kg.) 9,600 13, 900 Tensile strength retainingfactor after the shooting test: (dz/0' X100 (percent) 55. 6 79. 9

1 Siliceous sand with a known grain size distribution (Fig. 3A, curve 1,or Table 3, column 11) 2 Siliceous sand with a grain size distributionof the invention (Fig. 3A, curve 2, or Table 3, column III aRepresenting the load causing the pin breakbown.

Representing the load causing the porcelain breakdown.

Shooting distance: 45 feet Shooting angle: 45 with respect to the axisof suspension insulator, from below Gun and bullet: 22-caliber longrifle, high speed bullets Load applied on insulator: 10,000 poundtension The depth of radial cracks formed at the time of crashing in theradial direction extending from the plane 7a- 7b to the head portion 1of the porcelain, as shown in FIG. 1, was measured. Since more than oneradial cracks can be formed at the time of the crashing, the depth ofradial crack was represented by the maximum value of the lengths of theradial cracks as taken from the plane 7a-7b of FIG. 1 to the tip of thecracks extending toward the head portion 1.

The tensile strength of the suspension insulator after the shooting testwas determined by measuring the tensile strength of the suspensioninsulators, which were subjected to the aforesaid shooting test.

The values of various properties as tabulated in Table 6, are meanvalues of measurements made on seven test pieces and suspensioninsulators.

It is apparent from Table 6 that the toughness of the porcelaincontaining siliceous sand with a grain size distribution of theinvention is improved, as compared with that of the porcelain containingsiliceous sand with a known grain size distribution. The use of thesiliceous sand with the grain size distribution of the invention alsoresults in a considerable improvement in the crack-propagation-resistance of suspension insulators or in bulletresistancethereof. In other words, the length of radial cracks caused by shootingis greatly shortened in the suspension insulator made by using porcelainprepared by the method according to the present invention, as comparedwith that of known suspension insulators. Thus, thecrackpropagation-resistance is considerably improved.

EXAMPLE 2 The composition of alumina bodies used in Example 2 is shownin Table 7, including a body according to the present invention and aknown body.

The bodies of Table 7 were shaped and fired at 1,290 C. to produceporcelain test pieces, and the same bodies were used for preparingsuspension insulators by firing also at 1,290 C. Various properties ofthe porcelain test pieces and the suspension insulators were measured.The measurement of the properties in Table 7 was carried out in the samemanner as Example 1, except that the tensile load applied on thesuspension insulators during the shooting test was 15,000 pounds.

It is apparent from Table 7 that various properties, especially thetoughness and crack-propagation-resistance, of the porcelain andsuspension insulators can be greatly improved by adding the siliceoussand with the grain size distribution according to the presentinvention.

EXAMPLE 3 The composition of bauxite-containing bodies used in Example 3is shown in Table 8, including a body according to the present inventionand a known body.

TABLE 7 Porcelain Known of the Items porcelain invention Raw materialcomposition (percent by weight) Alumina 30. 0 30. 0 Fel'dspar 25.0 25.0Siliceous sand 10. 0 0. 0 Siliceous sand with a controlled grain sizedistribution 2 0.0 10.0 Gairome clay 35.0 35. 0

Total 100. 0 100. 0

Properties of porcelain:

Porcelain test pieces:

Toughness parameter: Z'YeE (X10 g.-

cm." 13.1 15.2 Youngs modulus: E (X10 g.-cm.c 10. 6 10.6 Fracturesurface energy: 'Ye (g.-cm.'

cmr 8.0 10. 8 Modulus of rupture (unglazed) (kg/cmfl)- 1 510 1, 530

Amount of crystals (percent by weight):

Corundnm 27 28 Mullite 13 11 Quartz 3 8 Totai 43 47 Suspensioninsulators:

Tensile strength of sound insulator 0' (kg) 24, 500 24, 400 Depth ofradial crack caused by shooting mm. 20 4 Tensile strength of insulatorsafter shootingitest 0'2 (kg) 14, 800 20, 700 Tensile strength retainingfactor after the shooting test: (oz/n) X (percent) 60. 5 84. 9

l Siliceous sand with a known grain size distribution (Fig. 3A, curve 1,or Table 3, column II).

2 Siliceous sand with a grain size distribution of the invention (Fig.3A, curve 2, or Table 3, column III).

3 Representing the load causing the pin breakdown.

4 Representing the load causing the porcelain breakdown.

TABLE 8 Known Porcelain alumina of the Items porcelain invention Rawmaterial composition (percent by weight):

Alumina 30. O. 0 Calcined bauxite with a controlled grain sizedistribution 1 0.0 30.0 Feldspar 25. 0 25. 0 Siliceous sand 15. 0 15. 0Gairome clay 30. 0 30.0

Total 100.0 100.0

Properties of porcelain:

Porcelain test pieces:

Toughness parameter: live-l:

(X 2 cm!) 13.5 14.4 Young's modulus: E (X10 g.-cm.' 10.6 10. 3 Fracturesurface energy 'Ye (g.-cm.cm- 0- 8. 6 10.0 Modulus of rupture (unglazed)(kg./

cm?) 1,510 1,520 Suspension insulators:

Tensile strength of sound insulator 2 0' g. 24, 400 24, 300 Depth ofradial crack caused by shooting mm. 18 8 Tensile strength of insulatorsafter shooting test 3 0'2 (kg) 14,900 18,300 Tensile strength retainingfactor after the shooting test (dz/01))(100 (percent) 61.1 75. 3

1 Oalcined bauxite with a grain size distribution as shown in Fig. 3B. 2Representing the load causing the pin breakdown. 3 Representing the loadcausing the procelain breakdown.

The bodies of Table 8 were shaped and fired at 1,290" C. to produceporcelain test pieces, and the same bodies were also used for makingsuspension insulators by firing at 1,290 C. Various properties of theporcelain test pieces and the suspension insulators thus prepared weremeasured, and the results are shown in Table 8. The values of themeasured properties were obtained by the same method as Example 1,except that the tensile load applied to the suspension insulators duringthe shooting test was identical with that for Example 2.

It is apparent from Table 8 that the toughness of the porcelain preparedby adding calcined bauxite with the grain size distribution according tothe present invention is considerably improved, as compared with that ofknown alumina porcelain. It should be noted here that thecrackpropagation-resistance, as determined by the shooting test, wasparticularly well improved by the addition of the calcined bauxite withthe grain size distribution of the present invention. In other words,the use of the calcined bauxite of the present invention resulted in theshortened length of the radial cracks extending toward the head portionof the insulator porcelain, so that the tensile strength of insulatorsafter the shooting was remarkably improved.

EXAMPLE 4 Bauxite-containing bodies used in Example 4 are shown in Table9, including a known alumina porcelain body and a body according to thepresent invention.

The bodies of Table 9 were shaped and fired at l,290 C. to produceporcelain test pieces, and the same bodies were also used for makingsuspension insulators by firing at l,290 C. Various properties of theporcelain test pieces and the suspension insulators thus prepared weremeasured, and the results are shown in Table 9. The values of themeasured properties were obtained by the same method as Example 1,except that the tensile load applied to the suspension insulators duringthe shooting test was identical with that for Example 2.

It is apparent from Table 9, that the toughness of the porcelainprepared by adding calcined bauxite and siliceous sand, each having thegrain size distribution according to the present invention isconsiderably improved, as compared with that of known alumina porcelain.

As can be seen from Tables 6 to 9, the improved toughness of theporcelain, i.e., the improved crack-propagationresistance of theporcelain, is in good agreement with the increasedcrack-propagation-resistance of suspension insulators made of theimproved procelain, as determined 16 by the shooting test. In fact, byusing the suspension insulators made of the porcelains, according to thepresent invention, in actual power transmission lines, radial crackscaused by shooting are prevented from propagating into the head portionof the insulator.

TABLE 9 Known Porcelain alumina oi the Items porcelain invention Rawmaterial composition (percent by weight):

Alumina 30. 0 0. 0 Caleined bauxit size distribution 0.0 30.0 Feldspar25. 0 25. 0 Siliceous sand 2 10. 0 0.0 siliceous sand with a controlledgrain size distribution 3 0.0 10.0 Gairome clay 35. 0 35. 0

Total 100. 0 100. 0

Properties of porcelain:

Porcelain test pieces: Toughness parameter: /2' .,-E

(X1O g.- uni-g) 13.1 11. a Youngs modulus: E (X10 g.-cm 10.6 10. 5Fracture surface energy: (g.-cm.-cm- 8. 0 14. 3 Modulus of rupture(unglazed) (kg./

Amount of crystals (percent by weight) Corundum 27 Mullite 13 18 Quartz3 8 Total 43 51 Suspension insulators:

Tensile strength of sound insulator m (1nm.) 20 3 Tensile strength of ishooting test oz (kg) 14, 800 21, 200 Tensile strength retaining factorafter the shooting test: (oz/c X100 (percent) 60. 5 87. 2

1 The same as the footnote 1 of Table 8.

Z Siliccous sand with a known grain size distribution (Fig. 3A, curve 1,01' Table 3, column II).

3 Siliceous sand with a grain size distribution of the invention (Fig.3A, curve 2, or Table 3, column III 4 Representing the load causing thepin breakdown.

5 Representing the load causing the porcelain breakdown.

As a result, it is possible to minimize the risk of serious line faults,e.g., a line drop fault, due to porcelain breakdown caused by thepropagation of cracks.

As described in the foregoing, according to the present invention, thereis provided a method for manufacturing porcelain having an improvedcrack-propagation-resistance, as compared with that of porcelainsmanufactured by known methods. With porcelains having the thus improvedcrack-propagation-resistance, the quality of porcelains for insulatorscan be improved. Thus, the invention contributes greatly to theindustry.

Although the present invention has been described referring tosuspension insulators, the application of the method of the invention isnot restricted to suspension insulators alone, but the method can beapplied to the manufacture of various other insulators usable in highvoltage power transmission lines.

What is claimed is:

l. A method for manufacturing porcelain for electric insulators with ahigh crack-propagation-resistance comprising shaping, drying, and firinga dried body at a temperature of l,180 C. to 1,350 C. to produce aproduct containing crystallites of undissolved quartz material in asuflicient amount to enhance the crack-propagation-resistance of theporcelain, said dried body consisting of to by weight of feldspathicmaterial, 10 to 30% by weight of quartz, and 30 to by weight of claymaterial, said quartz material in said dried body having 17 a grain sizedistribution falling within the following limits:

Percentagebywei' ht or particles'with an e ective diameter smaller thanthe 2. A method for manufacturing porcelain for electric insulators witha high crack-propagation-resistance comprising shaping, drying, andfiring a dried bodyv at a temperature of 1,180 C. to 1,350 C. to producea product containing crystallites of undissolved quartz material in asuflicient amount to enhance the cra'ck-propagation-re sistance of theporcelain, said dried body consisting of 20 to 40% by weight offeldspathic material, to 25% by weight of quartz material, 20 to 55% byweight of clay material, and to 45% by weight of alumina, said quartzmaterial in said dried body having a grain size distribution fallingwithin the following limits:

Percentage by wei ht of particles with an e ective diameter smaller thanthe 3. A method for manufacturing porcelain for electric insulators witha high crack-propagation-resistance comprising shaping, drying, andfiring a dried body at a temperature of 1,180 C. to 1,350 C. to producea product containing undissolved mullite in the polycrystallineaggregates of corundum and mullite in a sufficient amount to enhance thecrack-propagation-resistance of the porcelain, said dried bodyconsisting of to 40% by weight of feldspathic material, less than 2.5%by weight of quartz material, 20 to 55% by weight of clay material, and10 to 60% by weight of calcined bauxite, said calcined bauxite in saidbody having a grain size distribution falling within the followinglimits:

Percentage by weight of particles with an effective 1 About.

4. A method for manufacturing porcelain for electric insulatorsaccording to claim 3, wherein said shaping, drying "and firing a driedbody at a temperature of 1,180 C. to 1,350 C.-produces a'productcontaining crystallites of undissolved quartz material in a suflicientamount to enhance the crack-propagation-resistance of the porcelain andwherein said quartz material in said dried body has a grain sizedistribution falling within the following limits:

Percentage by weight of particles with an eflective diameter smallerthan the value below Limit for Limit for coarse disfine distri-Eflective diameter (u) tribution bution 5. A method for manufacturingporcelain for electric insulators with a highcrack-propagation-resistance comprising shaping, drying, and firing adried body at a temperature of 1,180 C. to 1,350 C. to produce a prodnotcontaining undissolved mullite in the polycrystalline aggregates ofcorundum and mullite in a sufiicient amount to enhance thecrack-propagation-resistance of the porcelain, said dried bodyconsisting of 20 to 40% by weight of feldspathic material, 20 to 55% byweight of clay material, and 10 to 60% by weight of calcined bauxite,said calcined bauxite in said dried body having a grain sizedistribution falling within the following limits:

Percentage by weight of particles with an effective diameter smallerthan the 1 About.

6. Porcelain comprising the vitrified product produced by shaping,drying, and firing a dried body at a temperature of l,l C. to 1,350 C.to produce a product containing crystallites of undissolved quartzmaterial in a suflicient amount to enhance thecrack-propagation-resistance of the porcelain, said dried bodyconsisting of 30 to 50% by weight of feldspathic material, 10 to 30% byweight of quartz material and 30 to 60% by weight of clay material, saidquartz material in said dried body hav- 19 ing agrain size distributionfalling with the following limits:

Percentage by weight of particles with an efieetive diameter smallerthan the 7 value below 7 Limit for Limit for coarse disfine distri-Efiective diameter (u) tribution bution Percentage by weight ofparticles with an efiective diameter smaller than the value below Limitfor Limit for coarse disfine distri- Eflective diameter (p) tributionbution 8. Porcelain comprising the vitrified product produced byshaping, drying, and firing a dried body at a temperature of 1,180 C. to1,350 C. to produce a product containing undissolved mullite in thepolycrystalline aggregates of corundum and mullite in a sufiicientamount to enhance the craclc-propagation-resistance of the porcelain,said dried body consisting of 20 to 40% by weight of feldspathicmaterial, less than 25% by weight of quartz material, 20 to 55% byweight of clay material, and 10 to 60% by weight of calcined bauxite,said calcined bauxite in said dried body having a grain sizedistribution falling within the following limits:

Percentage by weight of particles with an effective diameter smallerthan the 1 About.

9. Porcelain according to claim 8 wherein said shaping, drying, andfiring a dried body at'a temperatureof- 20 1,180 C. to 1,350 C. producesa product containing" crystallites of undissolved quartz material in asufiicient amount to enhance the crack-propagation-resistance of theporcelain and wherein said quartz material in said dried body has againsize distribution falling within the following limits:

Percentage by weight of particles with an effective v 10. Porcelaincomprising the vitrified product produced by shaping, drying, and firinga dried body ata temperature of 1,180 C. to 1,350 C. to produce aprod:uct containing undissolved mullite in the polycrystalline aggregates ofcorundum and mullite in a sufficient amount to enhance thecrack-propagation-resistance of the porce lain, said dried bodyconsisting of 20 to 40% by weight of feldspathic material, 20-tobyweight'of clay material, and 10 to by weight of calcined bauxite, saidcalcined bauxite in said dried body having a grain size distributionfalling within the following limits:

Percentage by weight of particles with an effective diameter smallerthan the References Cited UNITED STATES PATENTS IQI R FER ES Kingery, W.D.; Pressure Forming of Ceramics, in

Ceramicv Fabrication Processes; New York, 1958, pp. 55-

Kingery, W. D.; Introduction to Ceramics; New York, 1960, pp. 435, 607and 625.

TOBIASJE. LEVOW, Primary Examiner W. R. SATTERFIELD, Assistant Examiner

